Begin With Science
by Aisling-Siobhan
Summary: [Tony/Bruce][Tony/Hulk] Pepper had said "fuck the man and get it out of your system, Tony", so Tony would. It wasn't hard to talk Bruce around, because Bruce wanted it as much as Tony did. They just hadn't expected the Hulk to want to get involved as well.


Because a few people said they wanted to read the alluded to Bruce/Tony scene, here it is. Enjoy!

"**Begin with Science**"

**Disclaimer: ** The Avengers, Tony, Loki, etc belong to Marvel, Stan Lee, et co. I make no money from this and own nothing, don't sue.

**Summary: ** [Tony/Bruce] Pepper had said "fuck the man and get it out of your system, Tony", so Tony would. It wasn't hard to talk Bruce around, because Bruce wanted it as much as Tony did. They just hadn't expected the Hulk to want to get involved as well.

**Warnings: ** Slash. Post-Avengers. AU. Tumblr prompt. PWP. Tony/Bruce. Tony/Hulk.

**Rating: ** NC-17.

**A/N**: The prequel, so to speak, of a fill for a prompt I saw on Tumblr. **Deuteronomy 21:15** is already posted and is pure Frostiron, if you'd rather read that one (and haven't already).

**Title:** Oscar Wilde, "An Ideal Husband": "Romance should never begin with sentiment. It should begin with science and end with a settlement."

_XXX_

**Words: ** 3,259

**Chapter 1**

Bruce had a habit of running his left hand through his hair when he was nervous. Being nervous was something Bruce was used to feeling after his accident, but since meeting the Avengers it had gotten easier and easier to feel safe more often than not.

Unfortunately, for the last three weeks Tony had been making Bruce increasingly nervous. It didn't help that watching Bruce muse up his hair was a turn on for Tony, because seeing Bruce like that made Tony think of other ways they could mess him up. Ways that included having Bruce on his back, panting and writhing with his hair stuck to his forehead with sweat or his hands pulling at it as he cried out beneath Tony for more. But so far Bruce had been resisting Tony's subtle attempts to seduce him.

Pepper had given him the go ahead. In her exact words, she had told her boyfriend, "fuck the man and get it out of your system, Tony", while rolling her eyes and folding her arms across her chest. She had kept up the pretence of being irritated long enough for Tony to grin back and wink at her, before she had pulled him into a quick kiss. "Don't freak him out though," she had warned as she pulled away, "we don't want to have to reconstruct the Tower."

"Again," Tony had agreed his grin widening.

So Tony was trying to take it slow. He couldn't do his usual thing of buying a girl a drink because Bruce didn't drink, and he didn't care that Tony was rich or famous, and he knew Tony was a playboy but thought he was committed to Pepper so the idea that Tony was making him breakfast and coffee and dinner as an attempt to seduce him hadn't yet crossed Bruce's mind. The flowers Tony had left on Bruce's lab desk had wound up in the kitchen, rearranged in a vase along with some strange grass looking things procured by Steve, the teams new dinner table centrepiece, so either Bruce wasn't a flowers kind of guy or he didn't get that they were from Tony _for him_. He didn't want to just turn up in Bruce's room, naked, in the middle of the night either for fear of getting smashed by the Hulk, but Tony was starting to run out of slow-and-steady ideas. Bruce hadn't eaten the chocolate Tony left him; Clint did. Tasha had taken off with the spa tickets Tony had booked for Bruce. Even Thor was starting to interfere, eating the romantic meals Tony had attempted to make for Bruce each night this week so far, because Bruce seemed to be avoiding being alone with him, even for meals.

"Fine," Tony huffed to himself.

There was no one in the kitchen. But Bruce was hovering in the living room, contemplating whether his hunger was worth being alone with Tony or not, running his hand through his hair anxiously. His stomach fluttered from nerves, but there was a clench or two of hunger as well, enough to push him forward. Together in the kitchen, the two men simply stared at one another.

"You're so hot you denature my proteins," Tony drawled. Panic made him speak without thinking, he was used to being taken aback by a pretty face, less so by their personality and brains, but he kept enough sense about him to make sure he was out of the kitchen before Bruce could think of responding.

The next morning, Bruce headed down to his lab. Tony had built him a separate workspace, so they could work together or work alone, and today Bruce needed to be alone. He had been up most of the night trying to work out what Tony might have meant by that comment, because he hadn't of been flirting, had he? No! No he couldn't have been. Tony had Pepper; Tony could have anyone he wanted, why would he want the Hulk?

Bruce snorted, trying to push the thought out of his mind. He had work to focus on. Director Fury was still waiting for the samples he had taken from their last 'magic' wielding supervillian. The paper Bruce was working on was due to be published in a week and he was still missing a lot of the data he needed, the stupid machine he was using having decided to break down last week and the replacement parts not arriving yet. But when he arrived at his desk, the machine was fixed and the samples were correlated with the last batch and his email was open, displaying the samples already forwarded to the Director.

Tony had left a sticky note right on the centre of the desk, with his normal chicken scrawl covering it. It read: "Do you have 11 protons?"

Bruce flipped it over, smiling softly in a rather disbelieving manner as he read what Tony had wrote on the back.

"Because you're Sodium fine."

That evening Bruce was just getting into bed when his phone beeped. It was a text from Tony, and Bruce read it while shaking his head fondly. He resolved to talk to Tony about this in the morning, because they were friends and Bruce liked Tony probably more than he should already so Tony didn't need to keep trying so hard to make Bruce like him. It still hadn't occurred to him that Tony might be attracted to the younger man. He read the text out loud, smiling to himself as he saved it and turned off his phone.

"Want to meet up so I can excite your natural frequency?" Bruce snorted, slipping between the sheets on his bed. "Really, Tony? Honestly."

Bruce didn't get the chance to talk to Tony the next morning because the Avengers were called to assemble. The Hulk was busy smashing everyone that crossed his path, but he paused mid-swing when Iron Man flew a little too close for comfort.

"Hey big guy!" Tony yelled, struggling to be heard over the din of the fight. "How about we make like the change of base law, with you on the bottom, and me on top?" Hulk only stared at him; head tilted a little to the left as his eyes narrowed in confusion, but Tony blew him a kiss and flew off before any other reaction could be given.

Tony was the first one to arrive back at the Tower, and as always Bruce was the last. The others had split up, headed back to their rooms or out for the breakfast, but Tony waited in Bruce's room, having bribed Jarvis with not doing unspeakable things to his voice control system into letting him inside. He was only wearing a pair of briefs, bright red and rather tight, with a large gold proton drawn across his ass. There were words written above the proton in gold letters, clear enough to be read from a distance.

Tony waited until he could hear footsteps in the hallway. Jarvis' voice announced Bruce's presence at the door to his own room, and then when the door swung open Tony turned around and bent over, putting his ass on display for the other scientist.

"Does my mass look big in this?" Bruce read, eyeing the red briefs and Tony's shapely behind with a flush on his face and wide brown eyes. "Funny, hmm, but, uh, I don't think this is appropriate."

"I wanted to show you my new underwear and get your opinion. Do you think my ass looks big, Brucie?" Tony turned around, winking once at the startled man whose eyes had strayed down to rest on Tony's covered cock, already hard and straining against the fabric of his briefs.

"You could have shown them to me while you weren't wearing them," Bruce told him, finally drawing his gaze away from Tony's crotch. "And no, your butt is fine."

"It's damn fine," Tony agreed. He was grinning wider now, and when Bruce turned back around, having scrounged up some spare trousers for Tony from his duffle bag behind the door he was met by the sight of a completely naked engineer sprawled across his bed. "You wanted to see them off of me, so here you go. What do you think?" He allowed his legs to fall open, hand purposely brushing against his own hard cock as he reached over to hand Bruce the red briefs.

The scientist took them, hand automatically reaching out and accepting the offering. His tongue flicked out to wet his lips and the blush he was sporting spread further down his face, to his neck and collarbone. "What are, hmm, what are you doing, Tony?"

The man gave a frustrated sigh. "I'm trying to seduce you. But you're making it so fucking hard. Literally! Look at it; you could cut diamonds with my cock." Tony and Bruce both glanced down, Tony looking and feeling rather sorry for himself and Bruce's face was flushed with arousal and nervousness made his hand brush through his hair once again.

Tony had a habit of making him nervous lately, and after hearing Tony admit his intentions out loud Bruce could finally understand some of Tony's behaviour over the last month. He was trying to seduce someone who wasn't interested, someone who didn't want to get into his pants or who threw themselves at the engineer; someone Tony had to work for, and the man tried but failed spectacularly. Or maybe Bruce just failed at noticing the attention he was receiving.

Either way, both men were on the same page now, and there was a little voice in the back of Bruce's head – the Hulk's voice – snarling, "Fuck him."

"I don't think this is a good idea, Tony. I like you, I really do and-"

"I like you too," Tony butted in. He cut Bruce off mid-sentence, continuing to talk when the man opened his mouth to speak a second time. "This doesn't have to change anything. If there aren't sparks, fine, if there are, fine. Whatever, you know. But Pepper knows about this if that's what you're worried about, so that's totally cool. We can still be friends afterwards and we'll always be science bros, so that's good too. There's nothing to worry about."

"I've never had sex with a man before," Bruce admitted softly. His shirt was half unbuttoned, slowly giving way to reveal the dark hair scattered across his chest, but his fingers froze as he awaited Tony's response.

"I'll take good care of you, babe," the engineer promised.

There were so many reasons to say no, so many reason why he shouldn't do this, and a couple of reasons why he shouldn't want to do this. But his shirt was on the floor now and his belt soon followed it, fly unzipped and shoes and socks kicked off, because as much as he wanted to be sensible and reasonable and safe, Bruce wanted it as much as Tony did, so he said yes and let Tony fuck him.

_XXX_

Steve had only meant to knock twice and see if Bruce wanted to join them for an early dinner. They had all missed breakfast because of the fight, but Steve knew he and the two assassins had eaten when they had arrived back. Thor was still eating, but he'd probably eat again if free food was offered to him. Bruce hadn't been seen in the kitchen since he had arrived back and Jarvis wouldn't tell him whether or not Bruce was hungry and Tony needed to be forced to eat, Steve knew that much about the other man.

But what Steve hadn't intended to do was open the door when he knocked softly. It swung inwards, just a crack, because Bruce obviously hadn't closed it properly behind him, but a loud moan from inside the room startled Steve enough that he had the door opened fully before he registered that it wasn't a sound of pain. He found himself then, red faced and bashful, but unable to look away. His eyes widened, taking in the sight of Bruce on his hands and knees, of Tony behind him, inside of him, thrusting forwards and backwards desperately, hands gripping hips roughly and Bruce's own clawing at the sheets beneath him.

There were hints of green running along the back of Bruce's neck, bulges of muscles in his forearms that shouldn't have been there normally, and when he looked up Bruce met Steve's gaze with eyes that were poisonous green. The soldier thought of warning Tony about the impending incident but his voice wouldn't work; his tongue was stuck to the roof of his mouth and his cock throbbed in his trousers. It was only because he'd had good manners beat into him (literally, being caught staring at anyone in the forties meant you were gonna get dragged into an alley and beat up) that Steve resisted the urge to reach down and palm himself to orgasm.

But he kept watching, even though he knew he had been caught, despite the danger the Hulk presented to everyone, because Bruce was beautiful beneath Tony, face flushed and absolutely wrecked looking. His throat was bared as he threw his head back with a moan, arching his back beneath the other man, and Tony was glorious as he took him, deep and desperate, slamming his hips hard against Bruce's ass, cock slipping deeper and deeper the loose Bruce got, the closer they came to the edge, until he was shoved all the way inside and they couldn't tell where one started and the other ended anymore. Bruce's hair was mused again, and Tony clenched one hand through it, dark strands tangled around his tanned fingers, and he pulled, wrenching the scientists head back until Bruce screamed mouth open and silent. Tony's mouth was on his throat then, biting down hard as green shot all across Bruce's skin, spreading fast like fire, and he was coming, squeezing tight around Tony's cock as his insides convulsed, body shaking from his orgasm.

And then he was shoving Tony away, pushing him off and over to the far end of the bed. It was fluid, like water spilling over the edge of a table before you could catch it. One second Tony was about to come, buried balls deep inside of his man-crush and the next he was on his back and Steve was gone, running back to the kitchen and not stopping to close the door behind himself.

Hulk was crouched on his hands and knees and the legs of the bed had snapped in half, crumbling beneath the weight of the green behemoth. His huge head turned slowly, eyeing Tony with narrowed green eyes, watching him pant while trying not to come all over himself, because if he wasn't going to get to come inside of Bruce, he was at least going to come _on_ him.

"Tin man?" Hulk grunted.

"Hey big guy. You like that huh?" The Hulk gave a slow nod. "Ok then, no smashing, deal?"

"Hulk deal," the other agreed. He turned around fully, sitting cross legged on the ruins of the bed. His cock was dark green and thick, the width of Tony's thigh (which was why he had insisted he top when Bruce had asked if he might instead; in case this had happened, there was no chance of it happening with the Hulk _inside of him_). Semen clung to his thighs and stomach, sticky and an off-white colour which was probably something to do with the radiation and maybe Tony shouldn't lick it off of the Hulk's cock like he wanted to because it mightn't be safe to eat.

Tony managed to resist that urge. Instead he crawled into the Hulk's lap, settling himself so that he was straddling the creature with his legs spread as far as they could go and then further until he nearly cried out from the burning pain that followed, almost like doing the splits for the first time in a long time by complete accident. He ended up with his legs up around the Hulk's waist, his cock pressed fully up against the Hulk's cock, huge even when soft, trapped between Tony and his own stomach. Hulk's hands were on his ass, fingers overlapping to create a seat and Tony used it to raise himself up so that his legs didn't have to stretch so far, and he bounced, rutting against the Hulk's dick, writhing in his lap and hands until he was panting again, chest heaving and blood rushing. One finger, large, almost the width of an average sized dick, pushed against his hole, not penetrating but rubbing, soft tentative brushes along the perineum and back down to press against his hole again, the nail catching against the rim and making Tony gasp. He let out desperate, broken noises as the Hulk continued to finger him, never once pushing further than Tony could bear, not hurting him, not stretching him, just rubbing until Tony couldn't take it anymore, crying and writhing from the creature's touch.

"Mine," Hulk snarled.

Tony groaned lowly, before latching onto one of the Hulk's pectorals with his teeth and biting down hard as he came. The Hulk came with him, a shout escaping him that probably echoed around this floor of the Tower and up into the next, and his release covered the mess Tony had already made of his chest and the Hulk's cock, ridges sticky but pressing against Tony in all the right ways even as come dripped down the sides and Tony wanted to lick it clean, suck and swallow until every piece of evidence was inside of him and he could taste nothing but Bruce on his tongue for weeks to come. He wondered if he'd even fit the Hulk's cock in his mouth, unlikely, but he wanted to try so very much it was a physical ache in his gut, a clenching in his ass as he imagined what it would feel like to stretch himself out so far with toys, enough so that he could seat himself on the Hulk's dick, slide down slowly on to it, inch by inch as the Hulk sat patiently beneath him and let Tony work them both, feel it filling up every bit of space inside of him, so full he couldn't breathe, so far inside Tony thought he might choke on it. But the Hulk was gone by the time Tony got his breath back, and he ended up sprawled out on top of Bruce instead who was panting just as heavily, red faced and fucked out.

"You might be a Physics major, but you're no Bohr in bed." Tony joked, running a hand through Bruce's hair, until it was as messy at the back as he liked and the fringe was stuck to his forehead with sweat.

Bruce chuckled softly, offering Tony a chaste kiss on the cheek before he flopped backwards completely, all energy spent, and content to lie there and bask. Both men smiled tiredly at each other before falling asleep like that, tangled up limbs and sticky with seed and Bruce absolutely destroyed by Tony's teeth marks.

And in the morning if Bruce decided that the damage to the room and the danger that the Hulk might decide it wants Tony more than Bruce ever could was too much, that Tony's body wouldn't be able to take the Hulk like Tony thought it could, well, they had agreed to stay friends, hadn't they?

**The End**

It was meant to just be porn but we ended up with a bit of backstory and some rubbish flirting. I hope you enjoyed it.


End file.
